Explaining why particular taxonomic groups are more diverse than others, and why some appear to exhibit explosive adaptive radiations during evolutionary history, has long been a goal of evolutionary biologists. The silverside fish genus Chirostoma (Atherinopsidae: Tribe Menidiini), is a highly diverse group restricted to the Mesa Central of Mexico. Chirostoma has previously been referred to as a classic example of an adaptive radiation, however this has not be adequately tested with modern methods. The objective of this multidisciplinary project is to use DNA sequence data to investigate the evolutionary relationships among the genera and species within the Tribe Menidiini, and to gather ecological and morphological data to specifically address the tempo and mode of diversification within Chirostoma.
This field and lab study will train a new generation of students in a variety of scientific disciplines, including molecular systematics and geometric morphometrics. Collaboration, education, and student training are key components of the project. Chirostoma represents one of the most economically and culturally important groups of fishes in Mexico, and the results of this study will be of interest to biologists and others. A website and a photographic key to the Menidiine silversides of Mexico will be developed, and will be particularly useful to scientists, resource managers, and government agencies interested in fish conservation in Mexico.
Accomplishments: Intellectual Merit The objectives of this study were 1) to examine the phylogenetic relationships among the genera and species in the Tribe Menidiini using mtDNA and nuclear DNA sequences and 2) to examine the role of body shape and dietary specialization in the diversification of the New World Silversides (Tribe Menidiini). Silversides (Atherinopsidae: Tribe Menidiini) are an ecologically and economically important group of North American fishes distributed from southern Canada to southern Mexico. Menidiine silversides occur in freshwater, estuarine, and nearshore marine habitats, often occurring in large schools in the upper portion of the water column. The group reaches its greatest diversity in the lakes on the Mesa Central of Central Mexico, where as many as eight species co-occur in a single lake. Four genera are currently recognized within the Tribe including Chirostoma, Labidesthes, Menidia and Poblana. Chirostoma is the most diverse group consisting of 20 species that occur in lakes and rivers of Central Mexico. Labidesthes contains a single species that is widely distributed in freshwater systems throughout eastern North America in the Great Lakes and Atlantic basins. Menidia contains seven species that primarily occur in estuarine and marine environments, in addition to freshwater. Finally, four species of Poblana are restricted to the volcanic crater lakes Alchichica, Chignauapan, Quechulac, and Preciosa, in the state of Puebla, Mexico. Most species are planktivores, however, some species, particularly the inland freshwater species of Central Mexico, also feed on small invertebrates or small fishes. Species of Chirostoma only occur in inland freshwater habitats of Central Mexico and have diversified in several inland lakes, including Lakes Chapala, Patzcuaro, Zirahuen, and Zacapu, where several silversides co-exist in species flocks. Inland freshwater habitats represent prime systems to test predictions regarding diversification and ecological opportunity. Silversides of the Central Mexican Lake region are interesting from a radiation perspective because they were believed to have diversified along body size and dietary specializations not seen amongst other members of the Tribe Menidiini. Furthermore, the lack of significant genetic differentiation coupled with large degree of morphological variation is intriguing. Overall, the results indicate that the Tribe Menidinii is monophyletic and several of the genera are monophyletic. Diversification rates are higher in freshwater groups of silversides in comparison to marine lineages. In addition, the species of silversides in the Central Mexican lakes have diversified in body shape along axes associated with dietary specialization. The freshwater populations show greater disparity of body shape than marine/estuarine species of silversides and it appears as through the invasion of freshwater habitats on the Central Mexican Plateau, a depauperate isolated environment, allowed for rapid diversification through ecological opportunity. This study resulted in the publication of six scientific papers, nearly two dozen scientific presentations, and four additional manuscripts nearing completion. Accomplishments: Intellectual Merit This project facilitated international collaborations with multiple scientists in Mexico and provided detailed information on the distribution and abundance of silversides in central Mexico. This type of information is particularly useful considering the conservation status of many of these species hasn’t been addressed in decades and recent collecting by the PI has indicated that many formerly abundant species are now uncommon, overfished, and even extinct from the Mesa Central and nearly all of the declines are due to anthropogenic impacts. This study was also important in terms of education and student training, as multiple personnel with varying degrees of scientific experience were trained on this project. First, two first generation female graduate students received training in molecular systematics and evolutionary biology. Three REU students obtained valuable research experience and training in systematics, morphometrics, and museum curation. Eight students participated in one or more research expeditions to Mexico. Finally, a teacher from a nearby high school was funded with an RET supplement and gained hands-on research experience in the laboratory. Furthermore, the RET participant also developed teaching materials and laboratory exercises for the classroom based on her research experiences on this project.
